1. Field of the Invention
This invention relates to a fuel nozzle assembly for a gas turbine engine, and more specifically, to such a fuel nozzle assembly for a gas turbine engine having a fuel tube and an air tube enclosing the fuel tube to define an annular air passage therebetween, the air tube and fuel tube being slideably engaged to prevent contamination of the air passage. The fuel nozzle assembly is constructed such that the air passage is readily accessible for cleaning.
2. Description of the Prior Art
A typical fuel nozzle assembly capable of separately delivering both air and fuel to a combustion chamber generally comprises a fuel delivery tube supported from one end and having a fuel nozzle tip with a conical surface secured to the other, and an air delivery tube, also supported by the same one end, the air delivery tube enclosing the fuel delivery tube in a spaced relationship to define therebetween an annular air flow channel. A swirl cap is threaded onto the free end of the air delivery tube and tightened so that a conical opening in the swirl cap sealingly engages the conical surface of the nozzle tip. The swirl cap is further provided with a plurality of small apertures equilangularly spaced around the center of the swirl cap for directing atomizing air from the air flow channel in a direction convergent to the fuel which exits the fuel nozzle tip in an outwardly diverging conical pattern.
As the air delivered through the assembly is primarily used only at ignition of the gas turbine engine to atomize the fuel, it is important to provide an atomizing air pattern which is predictable and delivers an atomized fuel-air mixture generally adjacent to either a flame cross-over tube or a spark ignitor, or both.
The fuel nozzle tip injects fuel in an outwardly diverging, generally conical, pattern. However, during low fuel flow, fuel pressure atomization is poor and air is introduced through the swirl cap to further atomize the fuel injected by nozzle. In such a manner, the conical pattern is altered to result in a nodular or 4-spoke spray pattern. This additional atomizing air is necessary during light-off ignition to provide greater atomization of the fuel as it is introduced through the nozzle to reduce unburned fuel emissions and to obtain better distribution of the air fuel mixture to insure that it is properly delivered to the turbine to propagate the combustion process in the turbine. After light-off ignition is complete, the atomization air is cut off and fuel only is delivered through the nozzle to continue the combustion process.
To ensure that the air flow atomizes the fuel stream to the nodular spray pattern desired during atomization, the air flow is channeled through apertures having the same geometric orientation as the opening in the fuel nozzle tip through which the fuel is directed. However, providing the fuel spray and air spray with similar flow characteristics has proved unnecessary to achieve the desired nodular spray pattern of the atomized fuel spray.
Conical surfaces are utilized in such prior art devices because the conical seal, once established, was thought to provide the best air-tight seal available. To make the conical seal a high quality, air-tight seal, however, it was necessary to apply a fine grinding paste to the conical nozzle tip prior to engaging the nozzle tip with the swirl cap. Further, and more seriously, the conical nozzle tip and swirl cap utilized in achieving such a sealing interface lead to the formation of gaps at the fuel nozzle/swirl cap interface during axial expansion of the air delivery tube. This causes severe deterioration in the ability of the fuel nozzle assembly to provide the desired atomized fuel spray characteristics. In addition, such gaps encourage the formation of contaminants which further deteriorate the performance of the fuel nozzle assembly. The prior art devices are also prone to the accumulation of deposits in the air delivery channel which tends to clog it and do not provide access to the air delivery channel for removing such deposits. One such prior art fuel nozzle having a conical engagement between the swirl cap and the fuel delivery tube is disclosed in U.S Pat. No. 4,154,056 entitled "Fuel Nozzle Assembly for a Gas Turbine Engine", issued May 15, 1979 and assigned to the assignee of the present invention.
The problems identified in the prior art devices may be traced to the fact that the temperature of the fuel flowing through the fuel delivery tube is generally about 100.degree. Fahrenheit. The temperature of the air in the space between the tubes, however, may reach 600.degree. Fahrenheit. Such a temperature difference between the fuel tube and the air tube often causes varied axial expansion of the fuel tube and air tube, resulting in a disengagement of the conical seal between the fuel nozzle tip and the air delivery tube, thus creating the above-mentioned gap at the sealing interface between the two. This gap provides an area where contaminants from the air flowing therethrough or carbon deposits caused by occasional reverse flow from the combustor, can accumulate to prevent the gap from resealing. The air tube itself may also become clogged with contaminants. During shutdowns, the conical seal interface may be contaminated by fuel oil from the nozzle tip.
As such, any gap between the air tube and the fuel nozzle tip provides an air leakage path that deleteriously affects the atomizing air distribution such that an unpredictable fuel-air pattern can exist which produces erratic and unpredictable light-off characteristics. If contamination of the air passage is severe enough, the flow of atomizing air may be completely cut off, preventing light-offs.
Further, once the fuel nozzle assembly of the known prior art is assembled and mounted in a combustion chamber of a gas turbine engine, it becomes extremely difficult to mechanically clean the air delivery channel and remove the contaminants which may be causing either leakage at the sealing interface or blockage of the air delivery pipe.